The current state of the art for foam blow molding consists of using chemical or physical foaming additives introduced into the extruder with a compatible resin and suitable nucleating agent, if required. By whatever approach is employed, the desired outcome is to create a single-phase fluid that is homogeneous while under the temperature and pressure conditions within the extrusion system. Upon exiting the extrusion system into the ambient environment at the die tip, the sudden drop in fluid pressure results in the gas coming out of solution from the once single-phase polymer melt and forming gas-filled cells at nucleation sites established at discontinuities inherent to the polymer blend or to those presented by the dispersed nucleating agent. The cells will ideally grow spherically and remain individual, yet numerous, as the extruded parison is manipulated and formed into its ultimate shape. This process is very dynamic, enduring extensional forces, continuously changing nonlinear material property changes, temperature and pressure gradients, varying diffusivity and solubility of the gas within the polymer, intra-cellular pressures, cell coalescence and destruction, etc. Generally, polymers or polymer blends must be employed which exhibit specific combinations of melt index, melt tension, strain hardening, etc., in order to be properly extruded, foamed, and made to conform properly to a pre-defined part shape. The process of manipulating, stretching, and blowing even very low pressure air, relative to ambient atmospheric pressure, into the parison to form the parison onto the mold cavity inherently flattens and elongates the cell structure away from their more ideal spherical or polyhedral shapes.
The state of the art in blow molding of foam is to supply enough gas to the polymer to create sufficient foam at the die tip to survive the transition from extruded parison to formed article, while retaining the maximum number of discrete foam cells so as to maintain the desired cellular structure and low product density. Due to changes in pressure, diffusivity and solubility of the gas in the resin as it cools, some cells will simply shrink in volume and even disappear as the gas may be reabsorbed into the resin during cooling and subsequent application of forming pressure. In general, internal cell pressures are reduced, causing the cells to shrink and buckle during part formation and cooling. As the parison is expanded to reach the walls of the mold, the cells tend to become flattened and elongated parallel with the surface of the part which, on a local basis, is referred to as the material plane. With the application of forming pressure to impose definition to the part, these flattened cells can become even more flattened. Generally, a fine balance between gas content, resin system, extrusion die geometry, extrudate temperature and extrusion rate, ambient air and mold temperatures, forming pressure, and time are used to maintain the conditions to ensure maximum residual foam cell volume. In general practice, the foamed resin must be treated very gently in order to maintain the foam's integrity throughout the process. The final part tends to be populated with generally flattened cells, many with collapsed cell walls. These cells, being slightly compressed and collapsed by forming pressure, intercellular pressure reduction, and cooling related material shrinkage, tend to have a flattened and buckled shape that is aligned generally parallel to the material plane. Such foamed structures tend to have poor material properties both in bending and normal to the material plane. If conditions are not carefully maintained many cells may collapse or rupture, creating open-celled foams that can result in surfaces that are excessively rough and/or porous.
U.S. Pat. No. 8,517,059 assigned to Kyoraku discloses a blow molding foam process but fails to disclose a sub-ambient pressure process of using internal vacuum and specific mold thermal boundary conditions to expand and manipulate the structure of the foamed part walls after the part is formed into its final shape and while still in a molten or semi-molten state.
U.S. Pat. No. 7,169,338 assigned to JSP discloses a method for blow molding polyethylene foams using physical foaming agents that also specifies drawing the air from the interior of the formed part for the purpose of allowing two opposing walls in close proximity to fuse together internally without trapped air pockets. This patent fails to make any mention of using internal vacuum in any way to modify or control the foam structure nor is there any mention of either spherical cells or variable density structures.
U.S. Pat. No. 8,535,598 discloses a method for producing low density polypropylene foams in which the statement is made that chemical foaming agents are insufficient to reduce densities to below 0.7 g/cm3, which relates to an expansion ratio of 1.29 times, and which is consistent with all known information prior to this invention. This patent also discloses a limitation of maximum part width to die diameter ratio of 1.5 times.
U.S. Pat. No. 7,014,801 entitled “Polypropylene Resin Hollow Molded Foam Article and a Process for the Production Thereof” describes a scheme for selecting and blending various propylene resins of differing properties to yield a range of foamable base resins for use with physical foaming with carbon dioxide gas. Also disclosed is a potentially multi-layered, foamed, hollow article made by co-extruding discrete layers into a single parison. No mention is made of manipulating the foam structure to create expanded cells, nor is there any mention of creating a multilayered structure from a homogeneous monolayer extruded parison.
What is disclosed is a method of blow molding using a sub-ambient pressure process providing parts with smooth interior surfaces that are as smooth as, or smoother than, most current production non-foamed or foamed parts.